The invention relates to photonic band gap devices having waveguides, to optical wavelength demultiplexers, optical filters, and optical switches, particularly for use in optical communications. It also relates to integrated optical circuits, nodes for an optical network, methods of transmitting data using integrated optical circuits, and to software arranged to control the integrated optical circuits, again particularly for use in optical communications.
Optical or microwave components making use of the concept of a photonic bandgap (PBG) are known. The definition of a photonic bandgap material is commonly accepted as being a material having a property that electromagnetic radiation, (such as light) of a range of wavelengths, is not permitted to exist when the light is incident on a given part of the material at a range of angles. This can be seen as a dip or gap in the wavelength response, hence the name xe2x80x9cbandgapxe2x80x9d. It is caused by interference effects arising from periodicity in the structure of the material. Any lattice structure in the material, at the molecular level or higher, can give rise to such bandgaps. Typically the periodic structure is made by sub-micron size patterns created by etching. PBG devices are also called photonic crystals, xe2x80x9ccrystalxe2x80x9d and xe2x80x9clatticexe2x80x9d being defined broadly as any material with a repeating structure, whether a molecular crystal lattice or a manufactured repeating structure, or other repeating structure.
In principle, bandgap effects can be seen in one dimensional, two dimensional and three dimensional forms. An example of a one dimensional form is a series of layers of different refractive index, such as dielectric film-based multiplexer or demultiplexer devices, or fibre bragg grating devices. Both are well known. Two dimensional devices have been proposed, in the form of waveguides created in the surface of a crystalline structure. Three dimensional devices can be seen as an extension of the two dimensional devices by making waveguides in any direction of the bulk of such a crystalline structure. The remainder of this document will be concerned with two and three dimensional devices.
Some examples of the range of applications will now be described briefly. It has been shown that perfect photonic crystals have application as reflectors for a wide range of applications ranging from antenna systems to their already current usage as reflective optical coatings. In general these applications assume that the crystal is being used as a complementary device in their application and as such is not an integral part of the device.
As photonic crystals have rejection bands which specifically forbid propagation, they also forbid spontaneous emission. By controlling spontaneous emission, or suppressing it completely, the opportunity to control and enhance the efficiency of optical devices such as light emitting diodes (LEDs), and lasers is enormous. Defects introduced into a photonic crystal have very particular properties and their frequency dependence and quality factor, (Q), amongst other properties, can be engineered to suit their intended application. Within LEDs, defects can be used as emitters with the surrounding PBG lattice suppressing propagation and enhancing the emission characteristics. Defects may be substitution, lacunar, or interstitial types. Substitution may involve changing the optical index, the size, or the shape of an element of the crystal lattice. The lacunar type involves removing an element.
Another application is in waveguides. Lines of contiguous defects in the crystal may form waveguides. They work on the principle that the defects allow a small band of wavelengths to be supported, and transmitted, within the wider band defined by the band gap of the photonic crystal. An advantage of such structures is that the waveguides can have a very small turn radius of the order of several wavelengths of the optical signal which compares favourably with a typical turn radius of the order of several millimetres, or even centimeters which would be required for traditional core-cladding waveguides described above, which rely upon total internal reflection. A second significant difference compared to conventional waveguides is that the range of wavelengths passed can be determined by the defects making up the waveguide, whereas conventionally, separate filters would be required. The compactness and greater potential for integration, arising from both differences, could be commercially significant, particularly for WDM (Wavelength Division Multiplexed) systems having tens or hundreds of wavelengths.
An example of the application of particular photonic crystals as waveguides, by introducing defects to give a band of transmission within the photonic bandgap is shown in Joannopolous, J. D., Meade, R. D., Winn, J. N., Photonic Crystals Molding the Flow of Light, Princeton University Press ISBN 0-691-03744-2, 1995, particularly chapter 5. A photonic crystal is sandwiched between parallel slabs of material having lower refractive index to contain the optical signal by internal reflection. The crystal is formed by providing a lattice in a dielectric material. The lattice is formed by lattice sites at which the dielectric properties of the medium are varied relative to the bulk properties of the dielectric material. The resulting latticed region is essentially opaque to the optical signal. A waveguide can then be formed by discontinuities in the periodic lattice, for example by omitting a contiguous set of lattice sites. This is termed a lacunar type defect. The lattice sites have been made from cylinders of dielectric material, separated by air gaps. Hence omitting a contiguous line of cylinders leaves a waveguide made from air. Bends of 90xc2x0 have been introduced into such waveguides, but still suffer some consequential insertion loss due to reflection, as shown in Mekis, A., Chen, J. C., Kurland, I., Fan, S., Villeneuve, P. R., Joannopoulos, J. D., xe2x80x9cHigh transmission through sharp bends in photonic crystal waveguides.xe2x80x9d Phys. Rev. Lett. 77, 3787 1996, and Temelkuran, B., Ozbay, E., xe2x80x9cExperimental demonstration of photonic crystal based waveguidesxe2x80x9d Appl. Phys. Lett. 74: ,4, 486-488 Jan. 25 1999.
Such devices also have light containment problems in the third or vertical direction and serious device integration, coupling and fragility problems. If a hexagonal lattice is employed rather than a square lattice then reflection at the bend still occurs and once again parasitic loss mechanisms are introduced into the system. By employing the inverse lattice, such that air holes are introduced into a dielectric material, then a similar waveguide can be formed by in-filling a chain of holes or by separating two pieces of similar crystal.
Such devices guide within the dielectric channel, with the added benefit that guiding is maintained within the periodic plane by total internal reflection, unlike the guide made from air. Compatibility with other semiconductor devices in terms of integration and coupling issues, is also improved. However these dielectric guiding devices also suffer from reflections at bends introduced into the waveguide. Applications for such devices include multiplexers, demultiplexers, and equalization devices.
U.S. Pat. No. 5,651,818, Milstein et al, discusses in the introduction a number of available techniques of manufacturing photonic band gap materials. U.S. Pat. No. 5,784,400, Joannopoulous et al, proposes to utilise two-dimensional photonic band gap materials in an optical device in the form of a resonant cavity.
It is known from U.S. Pat. No. 5,389,943, Brommer et al, to utilise the frequency selective transmission properties of such two-dimensional photonic band gap materials in a filter in which transmitted light is modified in frequency response by the optical transmission characteristics of the bulk properties of the material. Further disclosed is the active control of material forming the lattice sites, such as by the application of an external field, in order to modify the refractive index of material at the sites and thereby actively control the transmissive properties of the filter.
A separate development involving replacing the continuous line of defects by a non continuous, periodic chain of defects, is shown by Stefanou, N. and Modinos, A., xe2x80x9cImpurity bands in photonic insulatorsxe2x80x9d Phys. Rev. B 57, 12127 1998, and by Yariv A, Xu Y, Lee R K, Scherer A, xe2x80x9cCoupled-resonator optical waveguide: a proposal and analysisxe2x80x9d Opt. Lett. 24: (11) 711-713 Jun. 1, 1999. The defects are lattice sites that have been either completely or partially in-filled, and the coupling properties of the defects can be used to form coupled resonance optical waveguides, (CROWs). The defects are located discontinuously through the lattice, but sufficiently close to each other to provide coupling between overlapping evanescent defect modes. Light can still be guided within the crystal and there is an advantage over contiguous defect waveguides that mode mismatch at corners is easier to manage with less consequential bend reflection loss. This is because sharp bends can take advantage of peaks in the coupling efficiency of a given defect at particular angles. To take full advantage of this, it is necessary to choose a defect type and an angle of bend in the chain of defects, such that there is a peak in coupling efficiency aligned to the direction of neighbouring defects in the chain in the lattice. This is predictable based on the crystal""s inherent lattice symmetry.
Experimental verification of wave guiding has successfully been demonstrated for various photonic crystals in the microwave regime by Bayindir M, Temelkuran B, Ozbay E, xe2x80x9cTight-binding description of the coupled defect modes in three-dimensional photonic crystalsxe2x80x9d, Phys. Rev. Lett. 84: 10, 2140-2143 Mar. 6 2000.
So far such discontinuous defect photonic bandgap devices have remained as academic discussion topics, and have not achieved widespread implementation. It has not been apparent how their properties will give rise to useful devices which can compete commercially with existing optical component technologies.
It is an object of the invention to provide improved apparatus or methods.
According to a first aspect of the invention, there is provided a photonic band gap device having a lattice structure, and having an electromagnetic waveguide, formed at least in part by a mesh of defects in the lattice, at least some of the defects being located discontinuously through the lattice, but sufficiently close to each other to provide coupling between overlapping evanescent defect modes.
The significance of the mesh of defects, is that it makes it easier to control the width and position of the transmission band, in wavelength terms, compared to a waveguide formed only from a planar defect, i.e. a single line of defects. Providing a mesh gives degrees of freedom in terms of e.g. centre frequency bandwidth, Q factor, by changing shape and configuration of the mesh and varying the types of defects. The term xe2x80x9cmeshxe2x80x9d is defined as any arrangement of defects which is not a single line, nor a solid block with no gaps. Unlike the underlying lattice, the mesh need not necessarily be a regular structure. It can be made up of lines of discontinuously located defects. It can also be made up of lines of continuously located defects separated by gaps. In this case, the gaps provide the discontinuous location of defects.
Compared to conventional waveguides based on dielectrics rather than photonic band gap technology, there is a significant compactness advantage. These advantages can be critical for applications such as optical devices for manipulating wavelengths in a wavelength division multiplex system. This is one application where, as the number of wavelengths increases, the physical space taken up by traditional components becomes prohibitive, and advantages of compactness achieved by integrating many devices and different types of devices, can become commercially very significant.
Notably, by using defects at discontinuous points in the lattice, mode mismatch at junctions and corners is easier to manage. Also, it is easier to define the band of wavelengths which is transmitted. The more widely separated defects, the higher the Q factor of the wavelength spectrum. In other words, a narrower transmission band with a sharper wavelength response can be achieved.
One preferred feature of some embodiments of the invention involves the mesh having a periodic structure. This can give better, less lossy transmission than non periodic structures. Where the mesh is formed from lines of defects, the periodicity can be a periodic structure or spacing between the lines of defects, or the periodicity can be in the spacing or type of defect within one or more of the lines.
Another preferred feature is that the waveguide is dimensioned to be suitable for transmitting optical signals. This is one of the most significant applications.
One preferred feature of some embodiments of the invention involves different parts of the waveguide being arranged to have different ranges of optical wavelength to be transmitted. This enables a variety of useful devices to be envisaged, such as wavelength filters, splitters, and wavelength multiplexers or demultiplexers for example.
Another preferred feature involves providing one or more junctions with other waveguides. The other waveguides can also be formed by a mesh of defects, and can be arranged to transmit different ranges of wavelengths. In particular, this enables devices such as multiplexers and demultiplexers to be produced. As the number of wavelength channels increases into the tens or hundreds, the advantages of greater compactness, and more integration over conventional optical components, become more commercially significant.
Another application is as a splitter, for dividing the optical power in a signal between two paths. This can find uses in creating redundant protection paths, or tapping off a small proportion of the signal for monitoring purposes. The same devices can be used in the reverse direction as couplers.
One way of making the waveguides support different ranges of wavelengths, is to have different separation distances between the defects. This can have a direct influence on the range of wavelengths transmitted. It can also influence Q-factor and finesse (that is, the width of a peak in the response curve, and the gradient at the sides of the peak). Other ways of affecting the range of wavelengths transmitted include altering the pattern or width of the mesh. Where narrow ranges of wavelength are required, such as at the demultiplexed outputs of a wavelength demultiplexer, a single line of defects can be used. A further way of differentiating the range of wavelengths, is to have different types of defect.
Another preferred feature of some embodiments is a tapered change in characteristics along the waveguide. This enables a change in effective aperture size for coupling to larger scale devices, e.g. optical fibre, or non photonic band gap planar waveguide type devices, and so on. This can be achieved by varying the structure of the mesh.
A further preferred feature of some embodiments is to provide wavelength selectivity by providing at least a portion of the waveguide in the form of a ring resonator. This has the advantage of having an easily definable wavelength response, since it depends directly on the length of the ring, which is relatively easy to design and manufacture accurately.
Another preferred feature of some embodiments is to provide active control of characteristics such as attenuation, to enable switching, or active control of the wavelength response, to enable devices such as adjustable filters, or dynamically reconfigurable add drop multiplexers. The active control can be provided either by changing the properties of the bulk of the crystal, or by changing the properties of just part of the waveguide, or of just the defects within a part of the waveguide (or any combination of these). This can mean using materials in the crystal or the defects, which are sensitive to alterations in temperature or electric field for example.
According to a second aspect of the invention, there is provided a photonic bandgap device having a lattice structure, and having an electromagnetic waveguide formed at least in part by defects in the lattice, the defects being close enough to provide coupling between overlapping evanescent defect modes, the waveguide being joined to at least one other waveguide, also formed by defects in the lattice, and having a wavelength response differing from the wavelength response of the first waveguide.
Again, as mentioned above, by using defects at discontinuous points in the lattice, mode mismatch at junctions and corners is easier to manage. Also, it is easier to define the band of wavelengths which is transmitted. The inventor has appreciated that the range of wavelengths transmitted can be defined differently in each of the joined waveguides, whether the waveguides are formed by meshes or otherwise. This makes use of the advantages of less mode mismatch and better definition of pass band to create useful devices including wavelength multiplexers, demultiplexers, wavelength filters, and so on.
A preferred feature of some embodiments is that the join can be an xe2x80x9cend to endxe2x80x9d join. Alternatively, it can be a xe2x80x9cyxe2x80x9d join, or other type of join including multi-way joins involving three or more waveguides. Another preferred feature of some embodiments is that either or both waveguides can be formed from a mesh of the defects. Another alternative is that either or both waveguides be formed from a line of the defects. The preferred or optional features mentioned above can equally be applied to this aspect of the invention.
Other aspects of the invention provide components, subsystems, network nodes, and networks incorporating such devices, and methods of using such devices, methods of controlling such devices. The preferred or optional features mentioned above can equally be applied to this aspect of the invention. Advantages other than those set out above, may become apparent to those skilled in the art, particularly over other prior art not known to the inventor.